In car design, ground effect is a series of effects that have been exploited in automotive aerodynamics to create downforce, particularly in racing cars, through underbody tunnels and floor design. This has been the successor to the earlier dominant aerodynamic focus on streamlining. The international Formula One series and American racing IndyCars employ ground effects in their engineering and designs. Similarly, they are also employed in other racing series to some extent; however, across Europe, many series employ regulations (or complete bans) to limit its effectiveness on safety grounds.
Theory In racing cars, a designer's aim is for increased downforce and grip to achieve higher cornering speeds. A substantial amount of downforce is available by understanding the ground to be part of the aerodynamic system in question, hence the name "ground effect". Starting in the mid-1960s, 'wings' were routinely used in the design of race cars to increase downforce (which is not a type of ground effect). Designers shifted their efforts at understanding air flow around the perimeter, body skirts, and undersides of the vehicle to increase downforce with less drag than compared to using a wing. This kind of ground effect is easily illustrated by taking a tarpaulin out on a windy day and holding it close to the ground: it can be observed that when close enough to the ground the tarp will be drawn towards the ground. This is due to Bernoulli's principle; as the tarp gets closer to the ground, the cross sectional area available for the air passing between it and the ground shrinks. This causes the air to accelerate and as a result pressure under the tarp drops while the pressure on top is unaffected, and together this results in a net downward force. The same principles apply to cars. The Bernoulli principle is not the only aspect of mechanics in generating ground-effect downforce. A large part of ground-effect performance comes from taking advantage of viscosity. In the tarp example above, neither the tarp nor the ground is moving. The boundary layer between the two surfaces works to slow down the air between them which lessens the Bernoulli effect. When a car moves over the ground, the boundary layer on the ground becomes helpful. In the reference frame of the car, the ground is moving backwards at some speed. As the ground moves, it pulls on the air above it and causes it to move faster. This enhances the Bernoulli effect and increases downforce. It is an example of Couette flow. While such downforce-producing aerodynamic techniques are often referred to with the catch-all term "ground effect", they are not strictly speaking a result of the same aerodynamic phenomenon as the ground effect which is apparent in aircraft at very low altitudes.
History
American Jim Hall developed and built his Chaparral cars around the principles of ground effects, pioneering them. His 1961 car attempted to use the shaped underside method but there were too many other aerodynamic problems with the car for it to work properly. His 1966 cars used a dramatic high wing for their downforce. His Chaparral 2J "sucker car" of 1970 was revolutionary. It had two fans at the rear of the car driven by a dedicated two-stroke engine; it also had "skirts", which left only a minimal gap between car and ground, to seal the cavity from the atmosphere. Although it did not win a race, some competition had lobbied for its ban, which came into place at the end of that year. Movable aerodynamic devices were banned from most branches of the sport. In 1968, the Argentine designer and engineer, Heriberto Pronello, developed the Pronello Huayra-Ford for the Sport Prototipo Argentino category, making its first appearance in Córdoba for the 1969 season with Carlos Reutemann and Carlos Pascualini as drivers. During 1968, a 1/5 scale model was made, which was tested in the wind tunnel of the Fábrica Militar de Aviones (FMA) usually employed by the Argentine Air Force, demonstrating the functionality of the ground effect at that scale. In 2023, the Pronello Huayra chassis #002 was invited to the Goodwood Festival Of Speed. During its stay in England, the car was taken to the Catesby tunnel, where a complete aerodynamic analysis was carried out by Argentine engineer and professor Sergio Rinland. "We always thought it had ground effect... When Heriberto tested it at the National University of Córdoba, he verified its air resistance with a 1/5 scale model that was perfect, without door and hood openings, without the intake turrets..." Rinland said. He further added, “The tests we did in the Catesby Tunnel demonstrated its great aerodynamic efficiency: we obtained a Cx 0.25 with the short tail and a Cx 0.23 with the long tail, which it used on the fastest circuits. Almost, almost what Heriberto had measured at the time” “It has a slippery upper shape and a flat floor with a diffuser that gave it quite an edge in its day. The diffuser has an expansion ratio that puts it staggeringly close to the maximum downforce you can get from a diffuser. The car was at the tunnel with pressure tapings added to it, in order to look at the pressure distribution around the car which looks to completely confirm that it works exactly as the designer expected.”, explained Willem Toet. These tests were carried out with and without the "long tail" which was used for high-speed circuits, with the vehicle propelled by its own means, at working temperature, returning consistent and repeatable results.
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